Paradise Falters for seed-eating birds

By Don Franklin

The tropical savanna boundary is home to many of Australia's seed-eating birds, with 55 of Australia's 90 species found there.¹ An alarming number of these birds are in trouble with problems proportionally more severe than for any other faunal group in the tropical savannas — and more severe than for seed-eating birds in other parts of Australia.

For example, in Queensland, the range of the gouldian finch has retreated 200 km northward on a front that is nearly 1000 km long. The gouldian finch has also disappeared from substantial areas in Cape York. During a survey at Pine Creek in the 1960s about 1000 gouldian finches were caught and banded in one week. In 1996, a similar survey was hard pressed to find half a dozen birds in a three-month survey.

In the face of a short period of European settlement in the savannas, serious problems for these birds have emerged. In contrast, the widespread species that dominate most seed-eating bird communities in Australia south of the tropics have mostly thrived despite what would appear to be much greater habitat degradation.

Is there a key to this pattern of change in the bird community? Using bird records collected throughout Australia for the Atlas of Australian Birds and a large historical database of records compiled from the literature and museum records there are some basic biogeographic questions about the birds that can be answered.

What we do know is that northern Australia is especially rich in seed-eating birds that feed exclusively on the ground, and outstandingly rich in finch species, with 14 of Australia's 18 species at home in the savannas. Thirteen species and 10 additional subspecies of seed-eating bird are found nowhere else in the world. (Click here for a list of seed-eating birds in the savannas)

Yet, and perhaps surprisingly, not one of these species can be said to be characteristic of the tropical savannas: they do not necessarily occupy all of the savannas, and many of them have restricted distributions.

Two, the buff-breasted button-quail and the Kimberley subspecies of the partridge pigeon have such small distributions and are so rare, difficult to identify or occupy such inaccessible terrain that you can count the number of documented records on your fingers and toes! Twelve of the taxa listed occur only in the Kimberley and Top End, and five in north Queensland only.

Classifications of seed-eating bird communities throughout Australia reveal a well-defined and sharp transition from the tropical savannas to the arid zone. But if you thought the Mitchell grasslands of the Barkly Tableland were tropical savannas, you obviously didn't ask the birds.

Birds found in this region have more in common with birds found in more arid parts of Australia rather than in the classic tropic woodlands. On the other hand, birds found in the tropical savanna communities are much closer to those of the eastern Australian woodlands, and the boundaries are less clear.

How so many species co-exist, and why so many have such restricted distributions, remains unknown. But that diversity may have been their undoing, for the more specialised a species, the more vulnerable it is to changes in its environment. Even a small shift in the nature of the ecosystem can mean certain species get pushed out of the system. The fact that so many seed-eating bird species appear to have evolved in the tropical savannas argues both for the considerable age of the savannas and for the diversity and reliability of the resources it provided. It is thought that the climate and eucalypt savannas became established about 15 million years ago.²

In the near future, I hope to be able to define the nature and extent of the problems of seed-eating birds in the tropical savannas with much greater clarity. In the process perhaps we'll find some clues to the real questions - why so many problems, and what can we do about them?

Don Franklin is a CRC researcher with the Parks and Wildlife Commission of the NT and has studied seed-eating birds of the savannas for the past two years.

These birds are unique to Australia's tropical savannas

• chestnut-backed button-quail
• buff-breasted button-quail
• partridge pigeon (both subspecies)
• white-quilled rock-pigeon (both subspecies)
• chestnut-quilled rock-pigeon
• northern rosella
• golden-shouldered parrot
• hooded parrot
• long-tailed finch
• masked finch (both subspecies)
• yellow-rumped mannikin
• pictorella mannikin
• gouldian finches

Plus sub-species of:

• squatter pigeon
• little corella
• sulphur-crested cockatoo
• Australian ringneck
• double-barred finch
• black-throated finch
• crimson finch
• star finch
• red-browed finch
• chestnut-breasted mannikin

Footnotes

1. These numbers vary a little depending on where you define the tropical savannas, and how you define a seed-eating bird.

2. Dunlop, C.R. & Webb, L.J. (1991). Flora and vegetation. pp 41-60 in Monsoonal Australia: Landscape, Ecology and Man. (Eds M.G. Ridpath, C. Haynes & M.A.J Williams). A.A. Balkema, Rotterdam, 231 pp. Pole, M.S. & Bowman, D.M.J.S. (1996) Tertiary plant fossils from Australia's Top End. Australian Systematic Botany 9: 113-126.